SaaS products pack a lot into every screen: sidebars, filters, tables, charts, settings panels, and status chips. Components repeat with small variations, so small token mistakes compound.
You also stack surfaces—page, section, card, nested panel—so contrast and hierarchy matter more than in a marketing landing page. There are more chances for muted text, borders, and state colors to fail at once.
That is why a theme system beats ad hoc styling for SaaS: you need one coherent model that scales as features ship.